1. Field of the Invention
The present invention relates to liquid crystal displays (LCDs). More specifically, the present invention relates to viewing angle reduction for Liquid Crystal Displays.
2. Discussion of Related Art
Liquid crystal displays (LCDs), which were first used for simple monochrome displays, such as calculators and digital watches, have become the dominant display technology. LCDs are used routinely in place of cathode ray tubes (CRTs) for both computer displays and television displays. Various drawbacks of LCDs have been overcome to improve the quality of LCDs. For example, active matrix displays, which have largely replaced passive matrix displays, reduce ghosting and improve resolution, color gradation, viewing angle, contrast ratios, and response time as compared to passive matrix displays.
FIGS. 1(a)-1(b) illustrate the basic functionality of a pixel of a vertical alignment LCD 100. For clarity, the LCD of FIG. 1 uses only a single domain. Furthermore, for clarity, the LCD of FIG. 1 is described in terms of gray scale operation.
LCD 100 has a first polarizer 105, a first optical compensation film 107, a first substrate 110, a first electrode 120, a first alignment layer 125, liquid crystals 130, a second alignment layer 140, a second electrode 145, a second substrate 150, a second optical compensation film 152, and a second polarizer 155. Generally, first substrate 110 and second substrate 150 are made of a transparent glass. First electrode 120 and second electrode 145 are made of a transparent conductive material such as ITO (Indium Tin Oxide). First alignment layer 125 and second alignment layer 140, which are typically made of a polyimide (PI) layer, align liquid crystals 130 vertically (or with a small pre-tilt angle) in a resting state. In operation, a light source (not shown) sends light from beneath first polarizer 105, which is attached to first substrate 110. First polarizer 105 is generally polarized in a first direction and second polarizer 155, which is attached to second substrate 150, is polarized perpendicularly to first polarizer 105. Thus, light from the light source would not pass through both first polarizer 105 and second polarizer 155 unless the light polarization were to be rotated by 90 degrees between first polarizer 105 and second polarizer 155. For clarity, very few liquid crystals are shown. In actual displays, liquid crystals are rod like molecules, which are approximately 5 angstroms in diameter and 20-25 angstroms in length. Thus, there are over 10 million liquid crystal molecules in a pixel that is 100 μm width by 300 μm length by 3 μm height.
In FIG. 1(a), liquid crystals 130 are vertically aligned. In the vertical alignment, liquid crystals 130 would not rotate light polarization from the light source. Thus, light from the light source would not pass through LCD 100; therefore, LCD 100 gives a completely optical black state and a very high contrast ratio for all color and all cell gaps. However, as illustrated in FIG. 1(b), when an electric field is applied between first electrode 120 and second electrode 145, liquid crystals 130 reorientate to a tilted position. Liquid crystals in the tilted position rotate the polarization of the polarized light coming through first polarizer 105 by ninety degrees so that the light can then pass through second polarizer 155. The amount of tilting, which controls the amount of light passing through the LCD (i.e., brightness of the pixel), is proportional to the strength of the electric field. Generally, a single thin-film-transistor (TFT) is used for each pixel. However for color displays, a separate TFT is used for each color component (typically, Red, Green, and Blue)
The viewing angle of LCD 100 is enhanced by optical compensation film 107 and 152. Specifically, the optical birefringence of liquid crystals 130 is positive because of the vertical orientation of liquid crystals 130 in the optical black state. This when, optical compensation films 107 and 152, have a vertically orientated optical axis and a negative birefringence, optical compensation films 107 and 152 compensates the oblique angle birefringence under crossed polarizer geometry. Furthermore, the on-axis optical transmission and contrast ratio for LCD 100 would not be reduced.
Additional LCD techniques to improve viewing angle include having multiple domains of liquid crystals. These types of displays are generally called Multi-domain vertical alignment liquid crystal displays (MVA LCDs). Optical compensation films are also used with MVA LCDs to further enhance the viewing angles.
However in many applications, a wide viewing angle is not desired. For example, in crowded areas, having a small viewing angle provides greater privacy to the user of the display as well as reducing disturbance to nearby people. Hence there is a need for a novel system or method to reduce the viewing angle of LCDs in a cost effective manner.